Former Guantánamo Bay detainee Binyam Mohamed has welcomed the announcement that the Metropolitan police have been called in to investigate whether MI5 agents were complicit in his torture, and promised to co-operate with the inquiry.

The attorney general, Lady Scotland, said in a written statement (pdf) today that she had given the allegations of possible criminal wrongdoing "very serious consideration" and felt there were sufficient grounds to launch a criminal investigation. But she stopped short of conceding a full judicial inquiry, which many critics have demanded.

"I'm very pleased that there's going to be an independent investigation," he told the Guardian today. "I remain concerned that the investigations shouldn't just focus on the small people and that one agent shouldn't be the scapegoat for what was a government policy. I understand that the investigation will include the people directly responsible for the torture, the Americans, and this is obviously very important."

The attorney general said a decision on criminal charges would be taken following the police investigation. Scotland said the evidence she had reviewed included "the open and closed judgments of the divisional court in the case; transcripts of all the evidence given by witness B; the other evidence and submissions made to the court; the foreign secretary's PII [public interest immunity] certificates; and material from third parties about the case."

Witness B was an MI5 officer who gave evidence in a judicial review in the high court last year. He told the court about a high-level policy on how to handle the interrogation of suspects.

He said: "I was aware that the general question of interviewing detainees had been discussed at length by security service management, legal advisers and government, and I acted in this case, as in others, under the strong impression that it was considered to be proper and lawful."

Following Scotland's statement, Jacqui Smith, the home secretary, said the security and intelligence agencies would "co-operate fully" with the investigation. "Wherever allegations of wrongdoing are made, they are taken seriously," she said.

Mohamed was questioned after being seized at Karachi airport in April 2002 travelling on a false passport. He was sent to an interrogation centre where, he says, he was hung up for a week by a leather strap around his wrists. Among his interrogators were officials from the US Central Intelligence Agency (CIA).

He was later visited by two British intelligence officers, one called John. The torture stopped when they came, Mohamed said. He said John told him: "I'll see what we can do with the Americans." He did not see John again.

Mohamed was flown to Morocco after being held incommunicado in Pakistan, where he was interrogated by an MI5 officer. From Morocco, he was rendered to Kabul's notorious CIA prison where he says he was held in darkness for weeks on end. He says that was the worst time in his seven years in US captivity.

MI5 telegrams to the CIA show security service officers fed the US with information on Mohamed when he was allegedly being tortured in Morocco. MI5 has said it did not know where he was or in what conditions because the CIA refused to say.

Among those calling for a judicial inquiry is Sir Ken Macdonald, the former director of public prosecutions, who said that only an inquiry would be "sufficiently transparent to attract public confidence".

"If crimes have been committed, to deal with them alone would amount to scapegoating and would, in any event, only scratch at the surface of the problem," he said.

Others have questioned the extent and rigour of Scotland's review.

Shami Chakrabarti, the director of the civil liberties group Liberty, said: "Whilst many will see the attorney general's announcement as coming better late than never, the five-month delay in reporting such a serious suspected offence to the police is far from an ideal example of respect for the law," she said.

"We look forward to the Metropolitan police investigation into this particular case but the wider public interest still requires a full judicial inquiry into all British involvement in extraordinary rendition."